Polyethylene terephthalate for molding and production the same

ABSTRACT

There is provided a method for producing a polyethylene terephthalate for molding which has a low content of cyclic trimer. The present invention is a method for producing a polyethylene terephthalate for molding which comprises (1) a condensation step of condensing bis(2-hydroxyethyl)terephthalate having an ion content of 10 ppm or less and an acid value of 30 mgKOH/g or less to produce an oligomer having an average polymerization degree of 4 to 10, (2) a melt-polymerization step of melt-polymerizing the oligomer to produce a prepolymer having an intrinsic viscosity of 0.50 to 0.65, and (3) a solid-state polymerization step of crystallizing pellets of the prepolymer and then solid-state polymerizing the prepolymer at a temperature of 190 to 230° C. to produce a polyethylene terephthalate having an intrinsic viscosity of not lower than 0.65; and a polyethylene terephthalate for molding which is obtained by the method and has specific properties.

TECHNICAL FIELD

The present invention relates to a production method of a polyethyleneterephthalate for molding. More specifically, it relates to a productionmethod of a polyethylene terephthalate for molding which usesbis(2-hydroxyethyl)terephthalate as a starting material and has a lowcontent of cyclic trimer. Further, the present invention also relates toa polyethylene terephthalate for molding.

BACKGROUND ART

A polyethylene terephthalate (hereinafter may be abbreviated as “PET”)is mass-produced and used as a material in a wide variety ofapplications such as fibers, films (including sheets), containers andelectric components, due to its excellent physical properties andchemical properties. Particularly, demand for PET bottles has beenrapidly increasing.

Properties required for PET differ according to applications. Forexample, a PET for bottles is required to be a polymer having a higherpolymerization degree than those for fibers for clothing and for films(including sheets) and to have low contents of cyclic trimer andaldehyde, from the viewpoints of bottle properties (such astransparency, toughness and flavor), moldability and other properties.

The PET for bottles is generally produced by subjecting terephthalicacid and ethylene glycol to an esterification reaction and then tomelt-polymerization so as to produce a prepolymer and subjecting theprepolymer to solid-state polymerization. The solid-state polymerizationis effective in achieving the above high polymerization degree and lowcontents of cyclic trimer and aldehyde. However, since these propertiesgenerally deteriorate during the melt molding process, the abovesolid-state polymerization effect is impaired during production ofbottles, and the required properties of bottles may not be satisfied.For example, a solid-state polymerized polymer for bottles which has anintrinsic viscosity of about 0.82 generally has a reduced content ofcyclic trimer of about 3,000 ppm, and when a bottle is produced byinjection-molding this polymer at 290° C., the content of cyclic trimeris increased to about 4,500 to 5,000 ppm during the melt moldingprocess. This content limits applications of the bottle and lowers theproductivity of the bottle.

The PET for bottles is generally produced by solid-state polymerizing amelt-polymerized prepolymer in a nitrogen atmosphere at temperaturesranging from 190 to 230° C. for about 20 hours. A primary reasontherefor is that even if the solid-state polymerization time isextended, an effect of reducing the cyclic trimer is small, it is verydifficult to reduce the content thereof to, for example, 2,500 ppm orless, and the intrinsic viscosity of the polymer becomes so high thatthe productivity of bottles decreases. This indicates that it isdifficult to satisfy conflicting properties, i.e., reducing the contentof the cyclic trimer by only solid-state polymerizing while theintrinsic viscosity of the polymer is kept within a range that securesexcellent melt moldability.

Thus, a method of suppressing by-production of cyclic trimer during themelt-molding process has been studied, and several proposals have beenmade. For instance, as a method of suppressing by-production of cyclictrimer during the melt-molding process, a method comprising bringing asolid-state polymerized polymer into contact with hot water of 50 to110° C. or water vapor of 70 to 150° C. to deactivate a polymerizationcatalyst (Patent Publication 1, Patent Publication 2, Patent Publication3 and the like) is proposed. According to this method, there is littledifference in the content of cyclic trimer between after solid-statepolymerization and after melt-molding at 290° C., and an example inwhich a bottle having a cyclic trimer content of 2,800 to 3,900 ppm canbe produced is disclosed.

Further, Patent Publication 4 discloses that a copolyethyleneterephthalate copolymerized with 0.5 to 3.0 mol % of isophthalic acid(hereinafter may be abbreviated as “IPA”) and 1.0 to 2.5 mol % ofdiethylene glycol (hereinafter may be abbreviated as “DEG”) and havingan intrinsic viscosity of 0.6 to 1.5 dl/g, a carboxyl end groupconcentration of not higher than 18 eq/ton and a cyclic trimer contentof not higher than 0.4 wt % is a polymer which by-produces a smallamount of a cyclic trimer at the time of melt-molding. As a specificexample thereof (example with the smallest amount of a cyclic trimer),it is described that when a solid-state polymerized copolyethyleneterephthalate copolymerized with 2.0 mol % of IPA and 2.0 mol % of DEGand having an intrinsic viscosity of 0.83 dl/g, a carboxyl end groupconcentration of 10.2 eq/ton and a cyclic trimer content of 0.23 wt %(2,300 ppm) was injection-molded at a cylinder temperature of 265° C., amolded article having a cyclic trimer content of 0.25 wt % (2,500 ppm)was obtained. However, this publication mentions nothing about thecontent of cyclic trimer when the polymer temperature is 290° C.

(Patent Publication 1) JP-A 3-47830 (the term “JP-A” as used hereinmeans an “unexamined published Japanese patent application”)

(Patent Publication 2) JP-A 8-231689

(Patent Publication 3) JP-A 6-234834

(Patent Publication 4) Japanese Patent No. 3072939

DISCLOSURE OF THE INVENTION Object of the Invention

The present inventor(s) has made intensive studies to develop apolyethylene terephthalate for molding which can produce a moldedarticle having a nearly equal cyclic trimer content to that of a polymerobtained by a hot water (or water vapor) treatment, without the hotwater (or water vapor) treatment of a solid-state polymerized polymer.As a result, the present inventor(s) has found that when a polyethyleneterephthalate (PET) is produced by use ofbis(2-hydroxyethyl)terephthalate (hereinafter may be abbreviated as“BHET”) having an ion content and an acid value within specific rangesas a starting material, the desired polymer can be obtained. The presentinventor(s) has found that according to the method, a PET for moldingwhich can keep the content of cyclic trimer after solid-statepolymerization to 2,000 ppm or lower and the content of cyclic trimerafter melt-molding to 3,500 ppm or lower can be obtained. Further, thepresent inventor(s) has found that the PET for molding shows excellentbottle moldability. Based on these findings, the present invention hasbeen completed.

Thus, an object of the present invention is to provide a method forproducing a PET for molding which has a low cyclic trimer content by useof BHET having an ion content and an acid value within specific rangesas a staring material.

Another object of the present invention is to provide a PET for moldingwhich shows (a) an intrinsic viscosity of not lower than 0.65, (b) aterminal carboxyl group concentration of not higher than 10 eq/ton and(c) a cyclic trimer content of not higher than 2,000 ppm aftersolid-state polymerization and (d) a cyclic trimer content of not higherthan 3,500 ppm after molten and kept at 290° C. for 30 seconds.

Other objects and advantages of the present invention will becomeapparent from the following description.

SUMMARY OF THE INVENTION

Firstly, the present invention is a method for producing a polyethyleneterephthalate for molding, comprising:

(1) a condensation step of condensing bis(2-hydroxyethyl)terephthalatehaving an ion content of 10 ppm or less and an acid value of 30 mgKOH/gor less to produce an oligomer having an average polymerization degreeof 4 to 10,

(2) a melt-polymerization step of melt-polymerizing the oligomer toproduce a prepolymer having an intrinsic viscosity of 0.50 to 0.65, and

(3) a solid-state polymerization step of crystallizing pellets of theprepolymer and then solid-state polymerizing the prepolymer at atemperature of 190 to 230° C. to produce a polyethylene terephthalatehaving an intrinsic viscosity of not lower than 0.65.

This production method includes, as a preferred embodiment, thesolid-state polymerized polyethylene terephthalate having a carboxyl endgroup concentration of 10 eq/ton or less and a cyclic trimer content of2,000 ppm or less.

The production method also includes bis(2-hydroxyethyl)terephthalatehaving an optical density of 0.000 to 0.010.

The production method also includes bis(2-hydroxyethyl)terephthalatehaving a purity of not lower than 95 wt %.

The production method also includes bis(2-hydroxyethyl)terephthalatecontaining 0.5 to 5 mol % of isophthalic acid based on an acid componentof bis(2-hydroxyethyl)terephthalate.

The production method also includes, as a preferred embodiment,performing condensation at a pressure of 7 to 70 kPa and a temperatureof 220 to 270° C. in the condensation step.

The production method also includes performing condensation in thepresence of a polymerization catalyst and a stabilizer.

The production method also includes the prepolymer having a carboxyl endgroup concentration of 10 eq/ton or less.

The production method also includes carrying out melt polymerizationeventually at a pressure of 25 to 140 Pa and a temperature of 270 to290° C. in the melt polymerization step.

Secondly, the present invention is a polyethylene terephthalate formolding, having:

(a) an intrinsic viscosity of not lower than 0.65,

(b) a terminal carboxyl group concentration of 10 eq/ton or less,

(c) a cyclic trimer content of 2,000 ppm or less, and

(d) a cyclic trimer content after molten and kept at 290° C. for 30seconds of 3,500 ppm or less.

This polyethylene terephthalate for molding includes, as a preferredembodiment, having a carboxyl end group concentration (b) of 6 eq/ton orless.

Further, the polyethylene terephthalate for molding includes having acyclic trimer content (c) of 1,000 to 1,800 ppm and a cyclic trimercontent after molten and kept at 290° C. for 30 seconds (d) of 2,500 to3,500 ppm.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the method of the present invention, there is provided amethod for producing a polyethylene terephthalate whose cyclic trimercontent is low and does not become higher than a given value even aftermelt-molding and is therefore suitable for molding.

More specifically, according to the present invention, there is obtaineda polyethylene terephthalate for molding which has (a) an intrinsicviscosity of not lower than 0.65, (b) a carboxyl end group concentrationof 10 eq/ton or less, (c) a cyclic trimer content of 2,000 ppm or less,and (d) a cyclic trimer content after molten and kept at 290° C. for 30seconds of 3,500 ppm or less.

BEST MODE FOR CARRYING OUT THE INVENTION

(Starting Material)

In the method of the present invention, bis(2-hydroxyethyl)terephthalate(BHET) having (A) an ion content of 10 ppm or less and an acid value of30 mgKOH/g or less is used as a starting material. This BHET has an ioncontent of preferably 5 ppm or less, particularly preferably 2 ppm orless, and an acid value of preferably 10 mgKOH/g or less, particularlypreferably 4 mgKOH/g or less. Further, the BHET preferably has anoptical density of 0.000 to 0.010, more preferably 0.000 to 0.006,particularly preferably 0.000 to 0.004.

Further, this BHET preferably has a purity of not lower than 95 wt %,more preferably not lower than 98 wt %. The purity of BHET is theproportion (wt %) of the BHET. Other components substantially include atleast one of an oligomer (having a polymerization degree of about 2 to10) of BHET, mono(2-hydroxyethyl)terephthalate (hereinafter may beabbreviated as “MHET”), 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (hereinafter may be abbreviated as “DEG ester”) anddicarboxylic acid.

For example, when a starting material is a mixture of 95 wt % of BHETand 5 wt % of terephthalic acid (hereinafter may be abbreviated as“TPA”) and/or isophthalic acid (IPA), the purity is 95 wt %. In thiscase, when the acid value of the BHET is 0 mgKOH/g, the acid value ofthe staring material is about 50 mgKOH/g. Further, when a startingmaterial is a mixture of 97 wt % of BHET, 1 wt % of DEG ester and 2 wt %of IPA, the purity is 97 wt %. In this case, when the acid value of theBHET is 0 mgKOH/g, the acid value of the staring material is about 20mgKOH/g. The BHET preferably contains isophthalic acid in an amount of0.5 to 5 mol %, more preferably 1 to 4 mol %, based on an acid componentof the BHET.

When the ion concentration of the BHET exceeds 10 ppm, the content ofcyclic trimer in a solid-state polymerized polymer becomes high, andwhen the acid value thereof exceeds 30 mgKOH/g, the content of cyclictrimer in the solid-state polymerized polymer becomes high undesirably.Further, when the optical density of the staring material becomes high,the color of the polymer is liable to deteriorate, and applications ofthe polymer may be limited. Thus, the optical density is preferably aslow as possible.

BHET in the present invention is not limited by a production methodthereof. BHET obtained by decomposing (depolymerizing) a waste PET,particularly crushed pieces of a collected PET bottle, with ethyleneglycol (hereinafter may be abbreviated as “EG”) to obtain adecomposition product solution (EG solution of decomposition product)and then subjecting this decomposition product solution to apurification treatment is preferred. In addition, BHET obtained by anesterification reaction of TPA and EG and BHET obtained by atransesterification reaction of dimethyl terephthalate (hereinafter maybe abbreviated as “DMT”) and EG are preferred. Of these, use of BHETobtained by decomposing (depolymerizing) a waste polyethyleneterephthalate (particularly crushed pieces of a collected PET bottle)with EG to obtain a decomposition product solution (EG solution ofdecomposition product) and then subjecting the decomposition productsolution to a purification treatment is particularly preferred.

To be more specific about the method using a waste PET, a waste PET,particularly crushed pieces of a collected PET bottle, is decomposed(depolymerized) with EG to produce a decomposition product solution (EGsolution of decomposition product). Then, this decomposition productsolution is subjected to a purification treatment to obtain BHET havingan ion content of 10 ppm or less, an acid value of 30 mgKOH/g or lessand a purity of not lower than 95 wt %. It is particularly preferred touse this BHET as a starting material.

In that case, the PET, particularly a PET used in a PET bottle (coloredPET bottle or transparent PET bottle), may be produced by any method.The PET may be a homopolymer or a copolymer, e.g., a copolymercopolymerized with a small amount (for example, 10 mol % or less basedon all acid components) of IPA, 1,4-cyclohexane dimethanol (hereinaftermay be abbreviated as “1,4-CHDM”), 1,4-butanediol (hereinafter may beabbreviated as “BD”) or the like. The optical density of the BHET ispreferably not higher than 0.010, more preferably not higher than 0.006,particularly preferably not higher than 0.004. The minimum value of theoptical density is 0.000.

The decomposition (depolymerization; glycolysis) reaction of the PET byuse of EG can be carried out by a conventionally known method andconditions, including the methods and conditions which have beenpreviously proposed by the present inventor(s), e.g., methods andconditions described in International Publications No. 01/10812 pamphletand No. 02/10117 pamphlet, JP-A 2002-121173 and the like.

To carry out this decomposition reaction (depolymerization reaction)efficiently, for example, the PET is first brought into contact with adepolymerization agent (preferably a distillation residue of crude BHET)composed essentially of BHET and/or a condensate thereof (preferably anoligomer having a polymerization degree of about 2 to 10) at an elevatedtemperature to carry out preliminary depolymerization. Then,depolymerization (actual depolymerization) is preferably allowed tofurther proceed by use of excess EG to prepare a decomposition productsolution containing crude BHET. This EG may be purified EG. However, itis also possible to use EG which contains a small amount of other glycolor EG which contains crude EG produced by the following crystallizationand solid-liquid separation.

As for the ratio of the amount of the PET to the amount of BHET (and/ora condensate thereof) in the preliminary depolymerization, the amount ofBHET (and/or a condensate thereof) is preferably 0.1 to 4.5 parts byweight, more preferably 0.1 to 2.0 parts by weight, particularlypreferably 0.1 to 1.0 part by weight, per part by weight of the PET. Thetemperature of the preliminary depolymerization is preferably 180 to290° C., more preferably 190 to 270° C., particularly preferably 200 to260° C. The reaction time is preferably 0.1 to 5.0 hours, morepreferably 0.3 to 1.5 hours. The decomposition product obtained by thepreliminary depolymerization preferably has a polymerization degree ofabout 5 to 40, more preferably 10 to 30.

The temperature of the depolymerization reaction (actualdepolymerization reaction) between the preliminary depolymerizationproduct obtained by the preliminary depolymerization and EG ispreferably 190 to 265° C., more preferably 200 to 220° C. As for theratio of the amount of the preliminary depolymerization product to theamount of EG, the amount of EG is preferably 0.5 to 8.0 parts by weight,more preferably 2.0 to 7.0 parts by weight, per part by weight of thepreliminary depolymerization product. When the amount of the preliminarydepolymerization product is excessively small with respect to EG, theamount of BHET to be produced becomes smaller than saturation solubilityin EG, and BHET can be obtained only in a smaller amount than themaximum yield obtained with respect to the total fluid volume to besubjected to a deionization treatment, which is uneconomical. Meanwhile,when the amount of the preliminary depolymerization product isexcessively large with respect to EG, an oligomer of BHET increases andthe yield of BHET decreases. Further, when BHET exists beyond thesaturation solubility of EG, BHET precipitates, so that the deionizationtreatment cannot be conducted. The depolymerization reaction time ispreferably 0.5 to 5.0 hours, more preferably 0.5 to 2.0 hours. Thepreliminary depolymerization product obtained by the depolymerizationcomprises BHET as a main component and can contain a small amount (forexample, 20 wt % or less, preferably 10 wt % or less based on allsolutes) of an oligomer having a polymerization degree of 2 to 20,preferably 2 to 10.

Further, when EG is used as the above depolymerization agent from thebeginning, the temperature of the decomposition reaction is preferably180 to 230° C., more preferably 190 to 220° C. The ratio of the amountof the PET to the amount of EG at the time of decomposition(depolymerization) is preferably 1:9 to 3:7 in terms of weight ratio.

The above decomposition reaction (depolymerization reaction) ispreferably carried out by setting a rectification column in adepolymerization reactor and distilling water out of the system from thereaction solution. In that case, it is preferable to allow evaporated EGto return into the system. By carrying out the decomposition process inthis manner, the amount of water in the decomposition product solutionwhen it is brought into contact with a cation exchanger can be renderedsmall, so that a hydrolysis reaction occurring with a decationizationtreatment can be controlled. It is preferred to adjust the amount ofwater contained in the EG solution to be brought into contact with thecation exchanger to 0.5 wt % or less. The amount of water can beobtained by measuring the decomposition product solution by use of KFmoisture meter MKC-510N of KYOTO ELECTRONICS MANUFACTURING CO., LTD.

The decomposition product solution obtained by the decomposition processgenerally comprises BHET as a main solute, EG as a main solvent, and anester of DEG which is contained in a PET as a raw material or producedby a side reaction at the time of the decomposition reaction(depolymerization reaction), as a secondary solute. Further, thedecomposition product solution may contain an oligomer of BHET or MHETas other solute component and DEG liberated from the DEG componentcontained in the raw material PET or the DEG component produced by theside reaction at the time of the decomposition reaction as a non-solutecomponent.

Further, the decomposition product solution may also contain a catalystused in the decomposition reaction (depolymerization reaction) (e.g., analkali compound such as caustic soda or caustic potash), a catalyst usedin a polycondensation reaction in the polyester producing step (e.g., anantimony compound such as antimony oxide, a germanium compound such asgermanium oxide or a titanium compound such as alkoxy titanium),additives such as a stabilizer (e.g., a phosphorus compound) and anantistatic agent, a colorant used in a colored bottle and the like(e.g., red iron oxide, carbon black, phthalocyanine blue or monoazoyellow), and impurity ions derived from various pollutions which aredifficult to expect.

The solvent used in the above decomposition reaction may contain a smallamount (preferably 10 wt % or less) of glycol other than ethyleneglycol.

The above decomposition product solution is preferably a solution havinga solid (solute) concentration of preferably 5 to 40 wt %, morepreferably 10 to 30 wt %, particularly preferably 17 to 23 wt %. Whenthe solid concentration of the solution obtained by the decompositionreaction (depolymerization reaction) does not satisfy this range, it ispreferred that the concentration be adjusted by use of EG.

The above decomposition product solution is preferably brought intocontact with activated carbon at a temperature of 50 to 100° C.,preferably 70 to 90° C., in its purification treatment. By thistreatment, a pigment (such as color index pigment yellow 151) having ahydrophilic group (such as a carboxyl group) may not be adsorbed andremoved in some cases, but other colorants are adsorbed and removed.This activated carbon treatment is also effective in removing componentswhich cause degradation in the performance of the ion exchanger.

The above activated carbon treatment of the decomposition productsolution can be conducted by passing the decomposition product solutionthrough a layer of activated carbon filled in a column or the like tomake them contact with each other, for example. When the decompositionproduct solution is a suspension, a blockage occurs in the activatedcarbon layer and drift occurs due to insufficient flow or flowresistance non-uniformity of the solution, so that a stabledecolorization treatment is difficult to conduct. Therefore, contactbetween activated carbon and the decomposition product solution ispreferably made after solid impurities of 1 μm or larger in size, e.g.,fine particles, are removed from the solution before and/or after theactivated carbon treatment, as required. The temperature of the solutionis preferably lower than or equal to the maximum working temperature ofthe ion exchange resin and is preferably a temperature at which crystalsof ethylene glycol ester of dicarboxylic acid, particularly BHET, arenot precipitated from the decomposition product solution.

Illustrative examples of the above activated carbon include coal-basedactivated carbon and wood-based activated carbon. Of these, thecoal-based activated carbon (e.g., “DIAHOPE008” of Mitsubishi ChemicalCorporation) is particularly preferred. Further, the shape of theactivated carbon may be powdery activated carbon, granular activatedcarbon or fibrous activated carbon, for example. Of these activatedcarbons, coal-based granular activated carbon is preferred from theviewpoints of an effect of removing impurities of the present inventionand strength in heating regeneration. The maximum diameter of theseactivated carbon (particles) is preferably about 1 to 3 mm.

The amount of the solution passing until the activated carbon undergoesbreakthrough is preferably 4,700 to 19,000 parts by weight based on 100parts by weight of the activated carbon, although varying depending onthe degree of pollution of the solution. The status of the breakthroughof the activated carbon can be known by determining the relationshipbetween the status of the breakthrough of the activated carbon and theOD value (optical density) of the decomposition product solution afterthe adsorption treatment in advance and measuring the OD value.

After brought into contact with the activated carbon, the abovedecomposition product solution is preferably brought into contact with acation exchanger and then with an anion exchanger. The deionizationtreatment of the decomposition product solution can be conducted bypassing the decomposition product solution through a layer of the ionexchangers filled in a column or the like to make them contact with eachother, for example. When the activated carbon treatment is carried outafter the ion exchange treatment, pigments and foreign matters which aresupposed to be adsorbed by activated carbon are adsorbed on the surfacesof the ion exchangers, thereby lowering ion exchange efficiencydisadvantageously. Further, when the decomposition product solution isbrought into contact with the anion exchanger first and then with thecation exchanger, an undesirable side reaction (such as production ofDEG or DEG ester) occurs in the decomposition product solution in thesubsequent step. The above cation exchanger and anion exchanger may bein the form of particles, chain or fibers or may be amorphous, forexample. When they are in the form of particles, thedecomposition-product solution can be brought into contact with them byfilling them in a column and passing the decomposition product solutionthrough the column.

When the above decomposition product solution is a suspension, ablockage occurs in the ion exchanger layer and drift occurs due toinsufficient flow or flow resistance non-uniformity of the solutioncomposition, so that a stable decolorization treatment is difficult toconduct, as in the activated carbon treatment. Therefore, contactbetween the cation exchanger and the anion exchanger and thedecomposition product solution is preferably made after solid impuritiesof 1 μm or larger in size, e.g., fine particles, are removed from thesolution after the activated carbon treatment, as required. Thetemperature of the solution is preferably lower than or equal to themaximum working temperature of the ion exchange resin and is preferablya temperature at which crystals of ethylene glycol ester of dicarboxylicacid, particularly BHET, are not precipitated from the decompositionreaction (depolymerization reaction) solution.

Since the maximum working temperature of the cation exchanger isgenerally higher than that of the anion exchanger, it is preferable tocool the solution resulting from the cation exchange treatment to themaximum working temperature of the anion exchanger or lower or to carryout the cation exchange treatment and the anion exchange treatment attemperatures lower than or equal to the maximum working temperature ofthe anion exchanger. In general, the proportion of cations in an ionimpurity is overwhelmingly higher than the proportion of anions.Further, the acidity of the decomposition product solution is lowered bythe cation exchange and a side reaction occurs in the subsequent step asdescribed below. Thus, the anion exchange treatment is preferablycarried out after the cation exchange treatment.

The above decomposition product solution is preferably contacted withthe cation exchanger for a residence time of 3 to 30 minutes, morepreferably 3 to 15 minutes. Further, contact between the decompositionproduct solution and the cation exchanger is preferably made at a spacevelocity of 1 to 12 hr⁻¹, more preferably 4 to 9 hr⁻¹. When thisresidence time is less than 3 minutes, the cation exchange treatmentcannot be carried out to a satisfactory extent, while when it is morethan 30 minutes, DEG or water is produced by the dehydration reaction ofEG. Thus, produced water promotes production of carboxyl group by thehydrolysis reaction of BHET and/or an oligomer thereof and the acidvalue of the reaction system increases disadvantageously. Further, atransesterification reaction between produced DEG and BHET causes thedegree of conversion into DEG ester to be a permissible value or largerdisadvantageously. In addition, after contacted with the cationexchanger, the decomposition product solution is preferably contactedwith the anion exchanger for 3 seconds to 10 minutes, more preferably 3seconds to 5 minutes, particularly preferably 3 seconds to 3 minutes.Thereby, the above dehydration reaction, transesterification reactionand hydrolysis reaction can be controlled.

As the above cation exchanger, a cation exchange resin is preferred, andas the above anion exchanger, an anion exchange resin is preferred.Specific examples of a cation exchange functional group in the cationexchange resin include —SO₃H and —COOH. Further, as the cation exchangeresin, those which are commercially available as DIAION SK1B, DIAIONSK104, DIAION SK110, DIAION SK112 and DIAION SK116 (products ofMitsubishi Chemical Corporation) and AMBERLITE IR120B, AMBERLITEIR120BN, AMBERLITE IR124 and AMBERLITE 200CT (products of Roam & HaasJapan) can be used, for example. In these commercial products, an ionexchange functional group is generally stabilized as a salt such as asodium salt. Thus, the functional group is preferably converted into afree acid radical as described above in using the commercial products.

Illustrative examples of the above anion exchange resin include thosehaving —N(CH₃)₂, —NH(C₂H₄NH)_(n)H, —N(CH₃)₃OH and the like as an anionexchange functional group. As these anion exchange resins, those whichare commercially available as DIAION WA10, DIAION WA20, DIAION WA21J andDIAION WA30 (products of Mitsubishi Chemical Corporation) and AMBERLITEIRA67, AMBERLITE IRA400J, AMBERLITE IRA96SB and AMBERLITE XE583(products of Roam & Haas Japan) can be used, for example.

Further, gel-type anion exchange resins are classified into a crackedtype and an uncracked type, and the uncracked type is preferred becauseless BHET is adsorbed. Further, a porous material, i.e., MR type(microporous type), which is an ion exchange resin showing betterphysical durability and a higher exchange adsorption rate than the geltype can also be used.

The maximum working temperature of the cation exchange resin is about120° C. in the case of a strongly acidic styrene-based cation exchangeresin and about 100° C. in the case of a weakly acidic methacryl-basedcation exchange resin. Meanwhile, the maximum working temperature of theanion exchange resin is about 40 to 60° C. in the case of a stronglybasic quaternary-ammonium-type anion exchange resin whose exchange groupis an —OH type, about 80° C. or lower in the case of a strongly basicquaternary-ammonium-type anion exchange resin whose exchange group is a—Cl type, and about 100° C. or lower in the case of a weakly basicprimary to tertiary amine (—NH₂R, —NHR₂ or —NR₃) type anion exchangeresin. Due to the above temperatures, it becomes preferable that thedecomposition product solution be cooled to a temperature of 40 to 60°C. and then subjected to the anion exchange treatment after subjected tothe cation exchange treatment at a temperature of 120° C. or lower. WhenBHET is precipitated by a decrease in the saturation solubility of BHETcaused by a decrease in the temperature, a proper amount of EG ofdesired temperature should be added. From an economical standpoint, itis preferable that the anion exchange treatment be carried out by use ofa primary to tertiary amine type anion exchange resin after the cationexchange treatment is carried out at a temperature of 50 to 100° C.,more preferably 60 to 95° C., much more preferably 70 to 90° C. In thatcase, the primary to tertiary amine type anion exchange resin ispreferably used in admixture with a cation exchange resin (preferably astrongly acidic cation exchange resin) since the anion exchange resin isdissociated in a neutral or acidic (preferably acidic) state and has anion exchange capability. The mixing ratio (volume ratio) of the aboveanion exchange resin to the above strongly acidic cation exchange resinis preferably 1:3 to 5:1, more preferably 1:2 to 3:1.

Illustrative examples of cations in the decomposition reaction productsolution include Na⁺ and K⁺ derived from the above decompositionreaction (depolymerization reaction) catalyst and Ca²⁺, Mg²⁺, Mn²⁺,Co²⁺, Zn²⁺, Sb³⁺, Ge²⁺ and Ti⁴⁺ derived from a catalyst or propertyimparting agent used in the polycondensation reaction in the polyesterproducing step. On the other hand, illustrative examples of anions inthe decomposition reaction product solution include PO₄ ³⁻ derived froma stabilizer and SO₄ ²⁻ and Cl⁻ which are ions adhered to thepolyethylene terephthalate. Since the amount of the cations issignificantly larger than that of the anions, it is preferable to carryout the anion exchange treatment after the cation exchange treatment.

Hydrogen ions are produced by the above cation exchange reaction, andthe treated solution shows being acidic. The produced hydrogen ionspromote the dehydration reaction of EG to produce DEG and water andpromote the transesterification reaction between BHET produced bydecomposition (depolymerization) of the PET and DEG to by-produce DEGester. Further, when the treated solution contains a large quantity ofwater, BHET undergoes hydrolysis to produce MHET. Further, thesereactions are further promoted when the solution is treated at hightemperatures ranging from, for example, 80° C. to 90° C. as comparedwith when the solution is treated at room temperature. Accordingly, thetime between completion of the cation exchange treatment and start ofthe anion exchange treatment is preferably as short as possible. Asdescribed above, the time is preferably 3 seconds to 10 minutes, morepreferably 3 seconds to 5 minutes, particularly preferably 3 seconds to3 minutes.

To control the transesterification reaction and hydrolysis reaction ofBHET, a method comprising neutralizing hydrogen ions by adding an alkaliis conceived. However, in this case, new cations derived from the alkaliare brought to the system, thereby negating the previously conductedcation removing treatment undesirably.

To control the transesterification reaction in the above cation exchangetreatment process, the residence time of the cation exchange treatmentis preferably shortened. While the ion exchange rate increases as thetemperature at which the ion exchange treatment is carried outincreases, the transesterification reaction rate also increases. Forthis reason, the residence time is shortened such that the degree ofconversion of the decomposition product by the transesterificationreaction becomes a permissible value or smaller.

In the above purification treatment, the transesterification reactionand hydrolysis reaction of the decomposition product obtained bydecomposing the PET with EG are controlled by carrying out the anionexchange treatment as quickly as possible after the cation exchangetreatment. Hydroxide ions are produced by the anion exchange treatmentand undergo a neutralization reaction with hydrogen ions, whereby thehydrogen ions in the reaction solution can be reduced.

The ion content of the decomposition product solution after undergoingthe ion exchange treatment is preferably 0.2 to 0.6 μS/cm, morepreferably 0.2 to 0.5 μS/cm, in terms of electric conductivity. Further,the pH of the solution is preferably 2.5 to 7.0, particularly preferably3.0 to 5.0. To render the electric conductivity lower than 0.2 μS/cm,the ion exchange treatment time must be increased, thereby increasingthe side reaction and making the pH smaller than 2.5, i.e., becomingcloser to the acidic side undesirably. Meanwhile, when the electricconductivity is higher than 0.6 μS/cm, the growth of precipitatedparticles in the crystallization treatment is inhibited, resulting insmall precipitated particles, and a decrease in yield in filtration anddeterioration in quality by remaining impurities occur undesirably. Theelectric conductivity can be measured by applying electric conductivitymeter 873CC of FOXBORO CORPORATION to the sample.

The decomposition product solution after undergoing the activated carbontreatment and the ion exchange treatment is preferably cooled to 15 to30° C. to carry out separation by crystallization of BHET as a soluteand remove a side reaction product and a coloring material which aresoluble in the EG solvent. A known evaporation distillation treatment ispreferably carried out thereafter to obtain high-purity BHET. It isconceived that in general, removal of a colorant and a coloring materialwhich could not be removed by the above pretreatment does not alwaysrequire the crystallization treatment and can be achieved by carryingout the evaporation distillation treatment. However, when the coloranthas sublimability or when the coloring material produced in thepretreatment step is to be prevented from entering the distillationtreatment step, separation by crystallization is effective. In thiscrystallization treatment, the size of precipitated BHET is preferably20 to 300 μm, more preferably 40 to 200 μm in terms of average particlediameter, and the precipitate is preferably solid-liquid separated bysolid-liquid separation means, preferably filtration equipment. Thisaverage particle diameter can be determined by measuring in a 10-folddilution by use of SALD-200VER of Shimadzu Corporation. Evaporation anddistillation of BHET are preferably carried out by a simple distillationtreatment or molecular distillation treatment. For example, moleculardistillation of BHET is preferably carried out at a pressure of 25 Pa orlower, more preferably 15 Pa or lower, and a temperature of 180 to 220°C., more preferably 185 to 205° C. By the molecular distillation,purified BHET with a purity of 95 wt % or higher or even 98 wt % orhigher can be obtained industrially advantageously.

When a waste PET, particularly a PET bottle, is used as a startingmaterial in production of the above BHET, it is very useful as aneco-friendly technology which makes chemical recycling of the waste PETpossible. Further, in some cases, as BHET, a product obtained from anesterification reaction between TPA and EG or a product obtained from atransesterification reaction between DMT and EG can be used. Thesereaction products can be purified by the above purification method asdesired. Further, they can be purified by a method described in thepamphlet of International Publication No. 01/10812.

BHET in the present invention, i.e., BHET having an ion content of 10ppm or less and an acid value of 30 mgKOH/g or less is first condensed(oligomerized) to produce an oligomer having an average polymerizationdegree of 4 to 10, the oligomer is then melt-polymerized to produce aprepolymer having an intrinsic viscosity of 0.50 to 0.65, and theprepolymer is then solid-state polymerized to produce a polymer havingan intrinsic viscosity of not lower than 0.65.

In that case, BHET as a starting material is preferably of singlecomposition. However, it may contain a small amount of a thirdcomponent, e.g., a dicarboxylic acid such as adipic acid, sebacic acidor IPA, a glycol such as 1,4-CHDM or BD, or the like, to adjustcrystallinity and the like. The amount of the third component variesaccording to the design of the properties of the final polymer. In thecase of a dicarboxylic acid, the amount of the dicarboxylic acid ispreferably such that the acid value of the whole starting material doesnot exceed 30 mgKOH/g. The amount of the dicarboxylic acid is morepreferably 0.5 to 5 mol %, particularly preferably 0.5 to 3 mol %, basedon the acid component of BHET. Meanwhile, in the case of a glycol, theamount of the glycol is preferably 0.5 to 5 mol %, particularlypreferably 0.5 to 3 mol %, based on the glycol component of BHET.

In the stage of the above oligomerization reaction andmelt-polymerization reaction, a reaction which produces DEG generallyoccurs as a side reaction. However, when the content of the DEGcomponent in the PET is high, the properties of the polymer deteriorate.Thus, this content must be made low.

Byproduction of DEG in the above reaction step is a reaction in which aterminal hydroxyethyl ester group and free EG in the reaction system areinvolved. Since these terminal groups exist in the reaction system in alarge amount in the initial stage of the reaction, the increase rate ofDEG is the highest at the start of the reaction. This indicates that thepresent invention using BHET as a starting material is in a moredifficult situation than the conventional method. Further, since DEG hasreactivity close to that of EG, is easily copolymerized and has aboiling point (about 245° C.) higher than the boiling point (about 198°C.) of EG, it is not easy to remove DEG from the reaction system bydistillation.

In the study made by the present inventor, it has been found that toreduce the content of DEG in the PET, it is more effective to remove EGfrom the system quickly so as to prevent production of DEG than toremove produced DEG. That is, to control byproduction of DEG by a director indirect reaction between the terminal hydroxyethyl ester group andEG, it is effective to remove free EG from the system as quickly aspossible in the oligomerization reaction stage. It has been found thatthe content of the DEG component in the produced polymer can be reducedby removing EG from the system quickly. Further, it has also been foundthat to further enhance the effect, it is effective to remove EG quicklyby heating EG not at normal pressure but under reduced pressure, sinceEG has a high boiling point as described above. Further, it has beenfound that in that case, it is important to prevent EG from refluxing orremaining in the reaction system. Further, control of byproduction ofDEG also controls byproduction of water, so that hydrolysis of BHET andthe produced oligomer can be reduced, production of carboxyl groups canbe inhibited and the acid value of the reaction system can be madesmall. The small acid value gives an effect of controlling byproductionof cyclic trimer. The oligomerization reaction and themelt-polymerization reaction in the present invention must be carriedout in consideration of the above side reaction.

(Condensation Step)

Therefore, in the present invention, oligomerization of BHET is carriedout by placing BHET in an oligomerization reactor and condensing BHETpreferably in the presence of a polymerization catalyst under a reducedpressure of 7 to 70 kPa, preferably 10 to 30 kPa at an elevatedtemperature of 220 to 270° C. while evaporating EG from the reactor toproduce an oligomer having an average polymerization degree of 4 to 10.In that case, it is preferable to prevent reflux of EG by heating theupper portion of the reactor and an EG distillation tube at the sametemperature as that of the inside of the reactor. When the abovepressure is lower than 7 kPa, bumping of the content of the reactoroccurs, while when it is higher than 70 kPa, the oligomerization takes along time, resulting in an increase in the amount of DEG. Further, whenthe above temperature is lower than 220° C., the oligomerizationreaction takes a long time, resulting in an increase in the amount ofDEG undesirably. The condensation reaction of the oligomerization ispreferably carried out in 30 to 90 minutes.

The above condensation reaction is preferably carried out in thepresence of a polymerization catalyst and a stabilizer. As thispolymerization catalyst, a known polymerization catalyst, e.g., antimonycompounds such as antimony trioxide and antimony acetate or germaniumcompounds such as germanium dioxide, can be preferably used. Thecatalyst may be dissolved in EG and added in the form of a solution ormay be dispersed in EG and added in the form of a dispersion. In thatcase, the concentration of the catalyst is preferably 0.1 to 20 wt %,more preferably 0.5 to 10 wt %. The condensation reaction can be startedafter heating without distilling out EG so as to cause thepolymerization catalyst to exert its function quickly by alcoholation ofthe polymerization catalyst. The heating time is preferably 10 to 60minutes, more preferably 20 to 60 minutes, particularly preferably 30 to60 minutes. The temperature is preferably 130 to 260° C., morepreferably 140 to 220° C., particularly preferably 150 to 200° C.

Meanwhile, as the stabilizer, phosphorus compounds such as phosphoricacid, phosphorous acid, dimethyl phosphate, dimethyl phosphate,trimethyl phosphate, trimethyl phosphite, triphenyl phosphate andtriphenyl phosphite, and amine compounds such as tertiary amines, e.g.,trimethylamine, tri-n-butylamine and benzylmethylamine and quaternaryammonium hydroxides, e.g., tetraethylammonium hydroxide,tetra-n-butylammonium hydroxide and trimethylbenzylammonium hydroxide,are preferably used. The amount of the polymerization catalyst may be aknown amount. For example, in the case of the antimony compound, itsamount is preferably 100 to 300 ppm, more preferably 150 to 250 ppm, interms of antimony element content, and in the case of the germaniumcompound, its amount is preferably 50 to 200 ppm, more preferably 80 to160 ppm, in terms of germanium element content. Meanwhile, as to theamount of the stabilizer, it is preferably 20 to 40 ppm in terms ofphosphorus element content in the case of the phosphorus compound, andit is preferably 1 to 100 ppm in terms of nitrogen element content inthe case of the amine compound.

(Melt-Polymerization Step)

Then, to obtain a prepolymer of desired polymerization degree, theinternal pressure of the system is decreased and the internaltemperature of the system is increased to carry out a polycondensationreaction of the above oligomer. This reaction is generally carried outin a melt-polymerizer which further increases the polymerization degreeof the oligomer. Further, the reaction to obtain the prepolymer may becarried out in a batch style in which the oligomer and the prepolymerare produced sequentially in the same reactor. For example, when thepressure is reduced to 25 to 140 Pa and the temperature is increased to270 to 290° C., preferably 272 to 285° C., a PET (prepolymer) having anintrinsic viscosity (mixed solvent of phenol/tetrachloroethane (1/1),25° C.) of 0.50 to 0.65, especially 0.52 to 0.63, can be formed. Thecarboxyl end group concentration of the prepolymer is preferably 10eq/ton or less, more preferably 6 eq/ton or less.

(Solid-State Polymerization)

The thus obtained prepolymer is cooled to be solidified into pelletshaving an average particle diameter of 1 to 5 mm. After predried asdesired, the pellets are crystallized to a specific gravity of 1.38 orhigher. Then, the crystallized pellets are solid-state polymerized at atemperature of 190 to 230° C. so as to obtain a PET having an intrinsicviscosity of not lower than 0.65.

The above predrying of the prepolymer is preferably carried out byheating the prepolymer at 60 to 100° C. for 4 to 12 hours. The abovecrystallization of the prepolymer is preferably carried out by heatingthe pellets to a temperature of 140 to 165° C. in 5 to 10 minutes in anitrogen atmosphere and then keeping the pellets under heating at 135 to165° C. for 5 to 15 hours. Since this crystallization treatment exhibitsan effect of drying the pellets, the water content of the pellets afterthe crystallization treatment is very low (generally 0.1 wt % or lower).Thus, the crystallized pellets can be directly subjected to solid-statepolymerization. When the water content of the crystallized pellets ishigh, the pellets are dried preferably in an inert gas atmosphere at atemperature of 120 to 170° C. over 3.5 to 7.0 hours before thesolid-state polymerization. After undergoing the above crystallizationand drying treatments, the prepolymer pellets are solid-statepolymerized preferably in an inert gas atmosphere or under vacuum at atemperature of 190 to 230° C.

As a result, a polymer (solid-state polymerized polymer) having anintrinsic viscosity of not lower than 0.65, a carboxyl end groupconcentration of 10 eq/ton or less and a cyclic trimer content of 2,000ppm or less is obtained. This polymer (solid-state polymerized PET)preferably has an intrinsic viscosity of 0.65 to 0.85, a terminalcarboxyl group concentration of 6 eq/ton or less and a cyclic trimercontent of 1,000 to 1,800 ppm. The DEG content of the solid-statepolymerized polymer is preferably 3 mol % or lower, more preferably 1 to3 mol %.

Further, the solid-state polymerized polymer in the present inventionhas a cyclic trimer content after molten and kept at 290° C. for 30seconds of 3,500 ppm or less, preferably 2,500 to 3,500 ppm. Further,when the solid-state polymerized polymer is brought into contact withhot water or water vapor of 50 to 150° C. to deactivate thepolymerization catalyst, an increase in the cyclic trimer content whenmolten and kept at 290° C. for 30 seconds can be 300 ppm or less. Inthat case, the time during which the polymer is in contact with the hotwater or water vapor is preferably 0.1 to 16 hours, more preferably 0.5to 8 hours.

Thus, according to the present invention, there can be provided apolymer for molding which is a solid-state polymerized polyethyleneterephthalate having:

(a) an intrinsic viscosity of not lower than 0.65,

(b) a terminal carboxyl group concentration of 10 eq/ton or less,

(c) a cyclic trimer content of 2,000 ppm or less, and

(d) a cyclic trimer content after molten and kept at 290° C. for 30seconds of 3,500 ppm or less, particularly 2,500 to 3,500 ppm. Further,a polyethylene terephthalate for molding which has been subjected to theabove hot water or water vapor treatment after solid-statepolymerization and shows an increase in the cyclic trimer content whenmolten and kept at 290° C. for 30 seconds of 300 ppm or less can also beprovided.

Properties in the present specification were measured in the followingmanner.

(1) Purity of Bis(2-hydroxyethyl)terephthalate

50 mg of sample was weighed precisely, about 100 ppm of solution wasprepared by use of chloroform, and the solution was analyzed by liquidchromatography (LC-6A of Shimadzu Corporation) to determine the amountof a monomer.

(2) Concentration of Carboxyl End Group

0.3 g of sample was dissolved in 30 ml of benzyl alcohol under heatingand then cooled. To the resulting solution, 20 ml of chloroform wasadded to dilute the solution. Then, by use of phenol red as anindicator, titration was carried out by a 0.01N-potassium hydroxidesolution to measure the concentration of carboxyl end group.

(3) Acid Value

This was measured by a neutralization titration method conforming to JISK0070.

(4) Optical Density

5 g of sample was dissolved in methanol so as to prepare a 10 wt %methanol solution. The absorbance at 380 nm of this solution wasmeasured by means of UVmini 1240 (product of Shimadzu Corporation) witha cell length of 10 mm and blanks zero-point corrected by use ofmethanol.

(5) Ion Content

The cation content was measured by use of SPS4000 of Seiko Denshi KougyoCo., Ltd. in accordance with inductively coupled plasma emissionspectrometry (ICP-AES), and the anion content was measured by use ofIC7000S of Yokogawa Electric Corporation and DX-300 of DIONEX CO., LTD.in accordance with ion chromatography.

(6) Average Polymerization Degree and Amount of Oligomer

5 mg of sample was weighed precisely, about 100 ppm of solution wasprepared by use of chloroform, and the solution was analyzed by liquidchromatography (LC-6A of Shimadzu Corporation) to determine the amountof an oligomer differing in the degree of polymerization.

(7) Intrinsic Viscosity of Polymer

A mixed solvent of phenol/tetrachloroethane (1/1) was used, a sample wasadded to a concentration of 0.4 g/100 ml, and the intrinsic viscosity ofthe polymer was determined at 25° C.

(8) Content of Diethylene Glycol

The content of diethylene glycol was measured by liquid chromatographyas in the above analysis of oligomer.

(9) Color of Polymer

The L, a and b of a polymer were measured by use of colorimeter NZ-Σ80of NIHON DENSHI KOGYO in accordance with a color hunter method.

EXAMPLES

Hereinafter, the present invention will be further described withreference to examples.

Example 1

(Condensation Step)

64.9 kg of high-purity bis(2-hydroxyethyl)terephthalate (BHET) having anacid value of 0.9 mgKOH/g, an optical density of 0.002, an ion contentof 0.5 ppm and a purity of 99.0 wt % and 0.9 kg of isophthalic acid(IPA) were mixed together to prepare BHET (acid value: 9.1 mgKOH/g,optical density: 0.002, ion content: 0.6 ppm, purity: 97.6 wt %) as astarting material and then charged into a reactor. Further, 12.0 g ofantimony trioxide, 2.1 g of cobalt acetate and 6.8 g of trimethylphosphate were also charged into the reactor. The inside of the systemwas substituted with nitrogen, the pressure inside the system wasreduced to 15 kPa by a vacuum pump, the internal temperature wasincreased from 120° C. to 235° C. over 25 minutes, and an oligomer wasproduced over 60 minutes while by-produced ethylene glycol (EG) wasdistilled out. The average polymerization degree of the obtainedoligomer was about 5.

(Melt-Polymerization Step)

Then, the oligomer was transferred to a polymerizer, and initialpolymerization was carried out by increasing the internal temperaturefrom 235° C. to 270° C. over 75 minutes and reducing the internalpressure to 40 kPa over 13 minutes, then to 100 Pa over 50 minutes andthen to 40 Pa or lower over 12 minutes. Then, the internal temperaturewas raised to 273° C., and final polymerization was carried out for 70minutes while the internal pressure was kept at 40 Pa or lower, therebyobtaining a polyethylene terephthalate (PET) having an intrinsicviscosity of 0.54.

(Solid-State Polymerization Step)

The obtained PET (prepolymer) was pelletized, kept in a nitrogenatmosphere at 135° C. for 10 hours to be crystallized and then chargedinto a rotary solid-state polymerizer, and solid-state polymerizationwas carried out under a vacuum of 40 Pa at 220° C. for 36 hours whilethe pellets were gradually rotated. The polymer resulting fromsolid-state polymerization had an intrinsic viscosity of 0.79, acarboxyl end group concentration of 0.3 eq/ton, a diethylene glycol(DEG) content of 1.9 mol %, a polymer color (b value) of 0.7, and acyclic trimer content of 1,630 ppm.

This polymer was injection-molded into a plate at a melt temperature of290° C. for a residence time of 30 seconds by use of an injectionmolding machine (Nissei Plastic Industrial Co., Ltd.: FN-1000-12A). Theobtained molded plate had a polymer intrinsic viscosity of 0.76, acarboxyl end group concentration of 12.0 eq/ton, and a cyclic trimercontent of 2,730 ppm. Thereby, it was revealed that this solid-statepolymer was at such a level that it could be used for a heat-resistantPET bottle.

Comparative Example 1

(Condensation Step)

45.4 kg of high-purity BHET used in Example 1, 12.7 kg of terephthalicacid (TPA) and 0.9 kg of IPA were mixed together to prepare BHET (acidvalue: 156 mgKOH/g, optical density: 0.005, ion content: 1.5 ppm,purity: 76.2 wt %) as a starting material and then charged into areactor. Further, 12.0 g of antimony trioxide, 2.1 g of cobalt acetateand 6.8 g of trimethyl phosphate were charged into the reactor, theinside of the system was substituted with nitrogen, and anesterification reaction (oligomerization reaction) was carried out atnormal pressure and a jacket temperature of 260° C. for 60 minutes whileby-produced water was distilled out. The average polymerization degreeof the obtained oligomer was about 5.

(Melt-Polymerization Step)

Then, the oligomer was transferred to a polymerizer, and initialpolymerization was carried out by increasing the internal temperaturefrom 235° C. to 270° C. over 75 minutes and reducing the internalpressure to 40 kPa over 13 minutes, then to 100 Pa over 50 minutes andthen to 40 Pa or lower over 12 minutes. Then, the internal temperaturewas raised to 273° C., and final polymerization was carried out for 70minutes while the internal pressure was kept at 40 Pa or lower, therebyobtaining a PET having an intrinsic viscosity of 0.59.

(Solid-State Polymerization Step)

The obtained PET (prepolymer) was pelletized, kept in a nitrogenatmosphere at 135° C. for 10 hours to be crystallized and then chargedinto a rotary solid-state polymerizer, and solid-state polymerizationwas carried out under a vacuum of 40 Pa at 215° C. for 20 hours whilethe pellets were gradually rotated. The polymer resulting fromsolid-state polymerization had an intrinsic viscosity of 0.82, acarboxyl end group concentration of 24.3 eq/ton, a DEG content of 2.5mol %, a polymer color (b value) of 0.9, and a cyclic trimer content of3,810 ppm.

By use of the obtained solid-state polymer, molding was carried out at290° C. under the conditions of Example 1. The obtained molded plate hada polymer intrinsic viscosity of 0.79, a carboxyl group concentration of29.7 eq/ton, and a cyclic trimer content of 4,090 ppm. Thereby, it wasrevealed that this solid-state polymer was at such a level that it couldnot be used for a heat-resistant PET bottle.

Comparative Example 2

(Condensation Step)

A mixture of 43.3 kg of TPA and 23.8 kg of EG was fed to anesterification tank having 1.0 kg of BHET charged therein in advance andkept at a temperature of 250° C. sequentially over 4 hours. Aftercompletion of feeding, an esterification reaction (oligomerizationreaction) was allowed to proceed to an esterification rate of 97% over 1hour.

(Melt-Polymerization Step)

Then, the reactants were transferred to a melt-polymerizer, 12.0 g ofantimony trioxide, 2.1 g of cobalt acetate and 6.8 g of trimethylphosphate were added, initial polymerization (melt polymerization) wascarried out at 275° C. under a reduced pressure of 2 kPa for 1 hour, andfinal polymerization (melt polymerization) was carried out at 277° C.under a reduced pressure of 50 Pa for 2 hours to obtain a PET having anintrinsic viscosity of 0.58.

(Solid-State Polymerization Step)

The obtained PET (prepolymer) was pelletized, kept in a nitrogenatmosphere at 135° C. for 10 hours to be crystallized and then chargedinto a rotary solid-state polymerizer, and solid-state polymerizationwas carried out under a vacuum of 40 Pa at 215° C. for 22 hours whilethe pellets were gradually rotated. The polymer resulting fromsolid-state polymerization had an intrinsic viscosity of 0.83, acarboxyl end group concentration of 24.0 eq/ton, a DEG content of 2.4mol %, a polymer color (b value) of −0.9, and a cyclic trimer content of4,070 ppm.

By use of the obtained solid-state polymer, molding was carried out at290° C. under the conditions of Example 1. The obtained molded plate hada polymer intrinsic viscosity of 0.80, a carboxyl group concentration of26.1 eq/ton, and a cyclic trimer content of 4,350 ppm. Thereby, it wasrevealed that this solid-state polymer was at such a level that it couldnot be used for a heat-resistant PET bottle.

INDUSTRIAL APPLICABILITY

The present invention is useful in the industry which produces and usesa polyethylene terephthalate to be molded which has a low cyclic trimercontent, particularly a polyethylene terephthalate for a heat-resistantbottle. Further, when a waste polyethylene terephthalate, especially bis(2-hydroxyethyl) terephthalate obtained by chemical recycling of PETbottles, is used as a starting material, PET-to-PET recycling can beachieved and is useful in future industries as an environment-friendlytechnology.

1. A method for producing a polyethylene terephthalate for molding,comprising: (1) a condensation step of condensingbis(2-hydroxyethyl)terephthalate having an ion content of 10 ppm or lessand an acid value of 30 mgKOH/g or less to produce an oligomer having anaverage polymerization degree of 4 to 10, (2) a melt-polymerization stepof melt-polymerizing the oligomer to produce a prepolymer having anintrinsic viscosity of 0.50 to 0.65, and (3) a solid-statepolymerization step of crystallizing pellets of the prepolymer and thensolid-state polymerizing the prepolymer at a temperature of 190 to 230°C. to produce a polyethylene terephthalate having an intrinsic viscosityof not lower than 0.65.
 2. The method of claim 1, wherein thepolyethylene terephthalate obtained by solid-state polymerization has acarboxyl end group concentration of 10 eq/ton or less and a cyclictrimer content of 2,000 ppm or less.
 3. The method of claim 1, whereinthe optical density of bis(2-hydroxyethyl)terephthalate is 0.000 to0.010.
 4. The method of claim 1, wherein the purity ofbis(2-hydroxyethyl)terephthalate is not lower than 95 wt %.
 5. Themethod of claim 1, wherein bis(2-hydroxyethyl)terephthalate contains 0.5to 5 mol % of isophthalic acid based on an acid component ofbis(2-hydroxyethyl)terephthalate.
 6. The method of claim 1, wherein inthe condensation step, condensation is performed at a pressure of 7 to70 kPa and a temperature of 220 to 270° C.
 7. The method of claim 1,wherein in the condensation step, condensation is performed in thepresence of a polymerization catalyst and a stabilizer.
 8. The method ofclaim 1, wherein the carboxyl end group concentration of the prepolymeris 10 eq/ton or less.
 9. The method of claim 1, wherein in themelt-polymerization step, melt polymerization is carried out eventuallyat a pressure of 25 to 140 Pa and a temperature of 270 to 290° C.
 10. Apolyethylene terephthalate for molding, having: (a) an intrinsicviscosity of not lower than 0.65, (b) a carboxyl end group concentrationof 10 eq/ton or less, (c) a cyclic trimer content of 2,000 ppm or less,and (d) a cyclic trimer content after molten and kept at 290° C. for 30seconds of 3,500 ppm or less.
 11. The polyethylene terephthalate ofclaim 10, wherein the carboxyl end group concentration (b) is 6 eq/tonor less.
 12. The polyethylene terephthalate of claim 10, wherein thecyclic trimer content (c) is 1,000 to 1,800 ppm, and the cyclic trimercontent after molten and kept at 290° C. for 30 seconds (d) is 2,500 to3,500 ppm.